The invention relates to improvements in taps, screws and analogous externally threaded bodies. Externally threaded bodies are characterized by projecting helical ribs winding around a cylindrical or conical shaft. The sizing of externally threaded bodies is based on the major diameter of the threaded portion which stands in certain relationship with the pitch of the thread.
Taps being used to cut an internal thread are available in different types, for example as machine-screw taps and hand taps. One subgroup of the latter are the serial hand taps which are normally sold in sets of two or three taps numbered, respectively, 1, 2, 3 with one, two, or three rings, respectively, on the shank. The No. 1 tap, called rougher, makes the roughing cut and is smaller both in major and pitch diameter. The No. 2 tap, called intermediate, cuts a somewhat fuller thread, and the No. 3 tap, called finisher, finishes the thread. In these sets of the serial hand taps each tap of set must be used in order to manufacture a finished threading, and all taps in the set belong to the same size.
It is well known to use screws or similar fasteners as a means for securing bars, rods, plates or other reinforcing, stiffening, immobilizing or aligning parts to broken or weak bones. Physicians specializing in the field of osteosynthesis employ such plates, bars, rods or like parts to ensure that the healing of a bone fracture will take place under optimal circumstances. This necessitates reliable attachment of plates or like parts (hereinafter referred to as plates) to the damaged bones, normally by means of so-called bone screws. The screws must be driven into a bone without any play to ensure that the plate that is being affixed thereby will maintain the joined fragments of a bone or two or more interconnected bones in optimum positions for rapid healing, i.e., in an optimum orientation of bones and/or bone fragments relative to each other.
As a rule, a plate that is to be implanted into the body of a patient and is to be secured to a tubular bone is provided with properly distributed holes for the passage of bone screws. The shanks of the screws are driven into the bone to locate one side of the plate adjacent the bone that is to be stabilized, reinforced or immobilized, and the heads of the screws are caused to bear against the opposite side of the plate. Those ends of the holes that are adjacent the opposite side of the plate are often configured in such a way that they receive portions of or the entire heads of the respective screws. A pattern can be used to drill holes into selected portions of a bone by resorting to a bone drill, and the surfaces surrounding the thus formed holes are provided with threads by resorting to a bone tap. Reference may be had, for example, to U.S. Pat. No. 4,943,292 granted Jul. 24, 1990 to Foux. The strength of the connection between the shank of a bone screw and a bone depends upon the relationship of dimensions of the screw thread on the tap and the dimensions of the screw threads of the bone screws.
The major part of the retaining action is furnished by the cortex or outer layer of the bone. Very little retention can be expected from the spongiosa and/or the tissue lining the marrow cavity. As a rule, a conventional screw that is driven into a tubular bone is subjected primarily or exclusively to tensional (pulling) stresses. In other words, care should be taken to ensure that such stresses do not result in extraction of the shank of a bone screw from the tapped hole in the bone. By far the major part of the retaining action is lost if the external thread is not a tight fit in the tapped hole of the bone. Attempts to eliminate such problems include the utilization of relatively long bone screws which are caused to pass all the way through a bone and mate with nuts on that side of the bone which faces away from the plate.
The problems are even more serious when a plate is to be affixed to one or more vertebrae, i.e., when one or more bone screws must be driven into the vertebra or vertebrae. Due to the anatomy of the spinal column, only one screw can be driven into a vertebra. On the other hand, a screw which is driven into a vertebra must often carry a very substantial load because the entire weight, or nearly the entire weight, of the upper part of the body must be borne by such screw. This creates problems, especially if a vertebra must be fixed in position with a very high degree of reliability, for example, in order to ensure desirable consolidation of the damaged spinal column. The situation is aggravated because the shank of the screw can be driven only into a certain portion of a vertebra, namely into the cortical parts of the pedicles because the remaining parts of the vertebra do not exert a pronounced retaining action. Reference may be had to German patent application No. 36 39 522 of Mattheck et al. (published Jun. 1, 1988). As mentioned above, a screw that is driven into a tubular bone (e.g., tibia) in an extremity of an animal body can be anchored in two cortices if it extends all the way through the bone. On the other hand a screw that is driven into a vertebra is anchored only once, namely in the narrow basal part called pedicle between the neural arch and the front part of the vertebra. The shape of a pedicle in sagittal section is that of a spool, and the shank of a screw can be reliably anchored only in the narrow central portion of such "spool" wherein the screw is capable of establishing and maintaining a satisfactory direct transmission of force as a result of tangential contact with the cortex of the respective portion of the vertebra.
A frontal cross-section through a pedicle has an elongated oval outline and can vary from one side of the vertebra to the other. This further complicates the insertion of a bone screw. Satisfactory fixation of a screw in a vertebra is a very difficult and complex procedure and is successful only when the external thread of the screw is in direct contact with the cortex of the pedicle in the region of the isthmus.
Still further, it is difficult, if not impossible, to invariably and reliably determine the maximum-diameter locus for the making of a tapped bore in a vertebra; this creates additional problems in connection with the selection of optimal bone drills, bone taps and bone screws. If, for example, the diameter of the initially chosen screw is too small there is no possibility to switch to a bone screw having a larger diameter without damaging the internal thread. As a rule, in externally threaded bodies the pitch is increasing together with the major diameter; screws for use as a tight fit in internal threads, i.e., without any play, as for example for use in wood, plastic and bones normally have a close relationship of major diameter and pitch. FIG. 6 shows portions of three conventional taps for the making of threadings with different major diameters C1, C2, C3; due to the fact that the ascending flanks of all threads exhibit the same angle gamma in relationship to the axis of the thread, the pitches h1, h2, h3 increase parallel to the major diameter. Consequently the threadings do not fit into each other anymore. For example, if such three taps are used one after the other to create an internal threading in a bore, the finally produced internal thread will have an irregular thread contour. In FIG. 7 a cross-section of such an irregular conventional internal threading containing a screw is displayed; there are many portions (blank areas) between the threadings in which the screw is not in direct contact with the internal thread. In addition, the internal threading has changed somewhat to an irregular hole; furthermore, this weakens the extraction resistance of the internal thread. If someone, to avoid the need to ultimately switch to a larger sized screw, initially selects a screw having an excessive diameter, such person will eventually face more severe problems due to the anatomical conditions around the neural arch when she or he penetrates the spinal foramen. Since the spinal cord and the roots of the nerves are immediately adjacent the pedicle of a vertebra, the shank of a screw that is driven into the vertebra may not extend from the isthmus of the pedicle.
Another factor that must be taken into consideration is that the frontal diameter of a pedicle varies within a rather wide range. In each segment of the spinal column, the difference between the maximal and minimal values can be as large as 6 mm. On the other hand, reliable and stable anchoring of the shank of a bone screw is conditioned upon proper selection of the dimensions of the tap and of the shank of the bone screw. If the dimensions of the shank of the bone screw are too small, the connection between the screw and the bone is unstable and the position of the implanted plate is not determined with the required degree of accuracy. Furthermore, too small screws are at risk of breakage, which is a very severe problem with bone screws of prior art in spinal fixation. If the dimensions of the screw are excessive, this often leads to neurological complications and can even result in paralysis. The invention relates to repair mechanisms for internal threadings which are worn out or are damaged by corrosion during use or have irregular flank profiles for other reasons, such as for example due to improper tapping. In accordance with the teachings of prior art, damaged internal threadings are treated in such a way that a new bore is drilled with a diameter being at least a little larger than the original major diameter of the internal threading in order to eliminate the damaged flanks of the old female threading. Thereafter, the enlarged bore is tapped to create a new internal threading which has a minor diameter larger than the original major diameter. The original screw fastener to be engaged in these new internal threadings has to be replaced too by one of larger size. In the cases where such a larger screw fastener is not acceptable, the so-called insert technique is applied, for example, by resorting to wire inserts of the type disclosed in U.S. Pat. No. 4,459,248. This insert technique does not eliminate the problems arising from the larger sized bore in material with internal threading. When the major diameter of the internal thread is enlarged by about 20 to 30 percent to be engaged by the insert, the remaining material around the internally threaded bore may be reduced to such an extent that as a result the holding power is seriously impaired; in critical cases the entire material block which holds these internal threadings has to be replaced.